Process for making a corrugation-free foam

ABSTRACT

A method for extruding a corrugation-free polystyrene foam with up to 100% carbon dioxide as the blowing agent by extruding the extrudate through an annular die opening so that it contacts a choke ring inner surface. A foam made with the apparatus according to the invention will have no visible corrugations and an average cell size of less than 0.35 mm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/418,165, filed Oct. 13, 1999, now U.S. Pat. No. 6,428,297, issuedAug. 6, 2002, which claims priority on U.S. provisional patentapplication No. 60/105,932, filed Oct. 28, 1998.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to polymeric foams. In one aspect, theinvention is directed to a process for producing a corrugation-free foamusing an annular die by extruding a mixture of a polymeric resin and ablowing agent through the die within a critical time. In another aspect,the invention relates to a process for making a fine cell size foam witha carbon dioxide blowing agent. The invention also relates to a processfor making foam by extruding a polymeric resin through a choke ring.

2. Background of the Invention

Low density polymeric foam, such as polystyrene foam, is conventionallymade by combining a physical blowing agent with a molten polymericmixture under pressure and, after thorough mixing, extruding thecombination through an appropriate die into a lower pressure atmosphere.This type of foam is commonly used to manufacture plates, bowls, cupsand like items.

From about the 1950's to the present, physical blowing agents of choicehave included halocarbons, hydrocarbons, specific atmospheric gases, orcombinations thereof. Examples of the halocarbons include commerciallyavailable halocarbon compositions such as dichlorodifluoromethane(CFC-12), trichlorofluoromethane (CFC-11) and mixtures thereof. Examplesof the hydrocarbon blowing agents are the C₂-C₆ alkanes such as ethane,propane, butane, isobutane, pentane, isopentane, and hexane. Examples ofthe specific atmospheric gases are carbon dioxide and argon.

During the 1980's, the worldwide scientific community presented evidencelinking halocarbons containing halogens other than fluorine, such aschlorofluorocarbons (CFCs) and hydrofluorocarbons (HCFCs) withatmospheric ozone depletion. Consequently, governments sought toregulate CFCs and HCFCs. As a result of such regulations, manufacturersof extruded polymeric foam products were forced to switch toalternatives which have had adverse effects that resulted in increasedprocessing costs, reduced product quality, and increased safety issues,or combinations thereof.

For example, hydrocarbon blowing agents, particularly the short-chainedalkanes produce foam with satisfactory physical properties. However,depending upon the location of the factory and the amount of the blowingagent used, a manufacturer may be required to capture and destroyemissions of the hydrocarbon blowing agents through a processing steplike incineration. Atmospheric emission of short-chained hydrocarbons,which are classified as photoreactive volatile organic compounds (VOCs),when combined with certain other gases and subjected to sunlight, mayresult in “smog”. Moreover, the flammability of the hydrocarbonsrequires elaborate control systems and costly ventilation systems toprevent the exposure of highly flammable blowing agent-and-air mixturesto ignition sources. Similar to hydrocarbon blowing agents, certainhydrofluorocarbon blowing agents, such as 1,1-difluoroethane (HFC-152a),produce foam with satisfactory physical properties, but have the adverseeffect of flammability. In addition, the nearly ten-fold higher unitpricing of these hydrofluorocarbon blowing agents in relation to most ofthe hydrocarbon blowing agents adversely increases foam product costs.

The disadvantages of the prior blowing agents have led to the use ofcarbon dioxide as a blowing agent. Carbon dioxide does not have theadverse environmental and flammability characteristics associated withCFCs and HCFCs. Carbon dioxide has a molecular weight that is lower thanmost of the commercially used hydrocarbons and the hydrofluorocarbonsand thereby requires lower usage rates. Carbon dioxide also has lowerunit pricing than the commercially used hydrocarbons andhydrofluorocarbons. However, the foams made with higher levels of carbondioxide have not been comparable to foams made with hydrocarbon or withhydrofluorocarbon blowing agents. The foams made with blowing agentsthat are primarily carbon dioxide have generally had both increasedproduct cost and decreased product quality. The increased cost isattributable to a combination of reduced extrusion rates and limitedpost-expansion in secondary operations, which results in increasedproduct weight. The reduced product quality is attributable to bothdiminished aesthetics and increased variability in the mechanicalproperties.

The diminished aesthetics of foam produced with carbon dioxide isgenerally related to larger cell size, often greater than 0.4 mm, whichgive such foams a poor, grainy texture, and to the presence of multiplevisible parallel regions of light and dark in the foam substrate. Theseadjacent parallel regions are not only deleterious to the visibleaesthetics of the foams, but also create significant localizeddifferences to mechanical properties.

The physical property that both diminishes aesthetics and increases thevariability of the mechanical properties of foams made with carbondioxide is related to the low solubility of the carbon dioxide gas inthe polymer at ambient atmospheric conditions. The low solubilityresults in a very high volumetric expansion rate of the foamablecomposition at the die. As a consequence of this high volumetricexpansion rate of the foam at the die, the use of a physical blowingagent comprised primarily of carbon dioxide in the production offine-celled foams having a foam density below about 100 kg/m³ or a cellsize below about 0.40 mm causes corrugation. The severity of thecorrugations tends to increase as either the density or the cell size isdecreased. The corrugations are manifest as periodic bands which areoriented in the machine direction within the extruded foam sheet andwhich differ in cell size, cell shape, and often foam thickness from themajority of the foam. The corrugations not only detract from theaesthetics but also reduce the overall mechanical properties of partsmade from the foam.

In most food service and beverage applications, it is preferred that theaverage cell size be about 0.20-0.30 mm, which provides the foam with anaesthetically pleasing, relatively smooth surface texture whilemaintaining the requisite mechanical properties strength. Smaller cellsizes tend to undesirably sacrifice a smoother finish for strength.Larger cell sizes tend to have an undesirable appearance.

When used as the sole blowing agent, carbon dioxide's very highvolumetric expansion rate typically produces unacceptable levels ofcorrugation. Therefore, previous attempts to use carbon dioxide as ablowing agent to produce a commercially acceptable foam product focusedon blending the carbon dioxide with another blowing agent. The blendedblowing agents typically included carbon dioxide as a minor constituentand either a hydrocarbon or hydrofluorocarbon blowing agent as thepredominant constituent. A common blended blowing agent would includecarbon dioxide in combination with pentane. Typically, the carbondioxide in the blended blowing agent was limited to 30 mole percent ofthe blended blowing agent, which reduced, but did not eliminate, the useof a hydrocarbon or hydroflurocarbon-blowing agent. Thus, the blendedblowing agent still has the disadvantages of the hydrocarbon andhydroflurocarbon blowing agents.

Attempts were also made to produce a commercially suitable polystyrenefoam with substantially 100 percent carbon dioxide as the blowing agent.Examples of such processes and foams are disclosed in U.S. Pat. Nos.5,266,605, 5,340,844, and 5,250,577. Most of these foams had an averagecell size of 0.36 mm and still contained visible corrugation. Althoughthese foams would be suitable for some applications, they did notproduce corrugation-free foams with cell sizes in the preferred range.

Referring to FIGS. 1 and 2, Applicants previously produced acorrugation-free polystyrene foam with 100 percent carbon dioxide as theblowing agent from a tandem extruder, which is commonly known in theindustry, in combination with a choke ring annularly positioned aroundan annular die extrusion opening.

The two-stage extruder apparatus comprises a hopper A feeding materialinto a first extruder B where polystyrene resin material is heated andmelted in heating zone C, mixed with a blowing agent delivered by aninjector D, further mixed by a mixing zone E, and cooled in a secondstage extruder in a cooling zone F before delivery to a die G. The chokering H contacts the exiting extrudate before a sizing mandrel I sizesthe sheet.

The previously-used choke ring 10 and die 12 are shown in more detail inFIG. 2. The choke ring 10 has a smooth temperature-regulated innersurface 11 that is positioned to be concentric with the die 12 so thatextrudate 13 contacts inner surface 11 before reaching the sizingmandrel 15.

The previously-used die 12 comprises a first generally convergingportion 16 that terminates at an annular die opening 18. A secondconverging portion 20 extends from the annular die opening 18 andterminates at a cylindrical portion 22.

The choke ring surface 11 and the annular die opening 18 are located aradius of r_(c) and r_(d), respectively, from the longitudinal axis 24.The choke ring gap is the difference between the radii (r_(c)−r_(d)).

Two choke rings having different diameters were tried. The first chokering had a diameter such that the gap was 0.2375 inches or 6.03millimeters. The second choke ring had a gap of 0.18 inches or 4.57millimeters, resulting in a contact time of approximately 0.37 ms forthe given operational parameters. Although both of these choke ringsproduced corrugation-free foam, the cell size remained above 0.40 mm.Therefore, there is still a need for a polystyrene foam and method ofmaking a polystyrene foam that is corrugation-free with a cell size inthe preferred range and using a carbon dioxide blowing agent.

SUMMARY OF INVENTION

The invention relates to a method for making ta orrugation-free foam.The foam is preferably made in an extrusion apparatus that extrudes apolymeric foam from an extrudate comprising a polymeric resin and ablowing agent. The blowing agent provides the extrudate with a cellularstructure upon extrusion. The apparatus can comprise an extruder havingan inlet that is adapted to receive the extrudate. An extrusion die isprovided on the extruder and forms an outlet for the extruder. Theextrusion die defines a longitudinal axis and has an annular die openingthat is concentrically oriented relative to the longitudinal axis andpositioned a first radial distance from the axis. The apparatus furthercomprises a choke ring having an opening defined by an annular chokering surface. The choke ring is positioned relative to the extruder suchthat at least a portion of the die is received within the choke ringopening. The choke ring opening is preferably positioned concentricallyabout the longitudinal axis and is positioned a second radial distancetherefrom. The difference between the second radial distance and thefirst radial distance defines the gap between the choke ring and thedie. The foam is extruded from the extrusion die through the choke ringat a preselected line speed. As the foam leaves the extrusion die, itcontacts the choke ring during an interval of time termed the contacttime.

The contact time is a function of the line speed and can becharacterized by the ratio of the gap size in millimeters (mm) relativeto the line speed in millimeters per second of the extrudate as itleaves the die opening with the ratio being between 0.001 and 0.020seconds. The choke ring gap is less than 4.6 mm and is preferably lessthan or equal to 0.8 mm. The gap can be specified as the differencebetween the first and second radial distances. It can be specified interms of the combination of a selected line speed and a ratio valuebetween 0.001 and 0.020 seconds. The gap size can also be specified interms of a selected contact time for a given range of line speeds.

The die can be of any suitable shape; however, it is preferred that thedie is symmetrical relative to the longitudinal axis. It is alsopreferred that the die has a generally cylindrical body with an annularslot defining the die opening. The generally cylindrical body caninclude an annular ridge extending from the cylindrical body andterminating in a peak in which the annular slot is located.

In the method according to the invention, a polymeric foam is extrudedfrom an extrudate that comprises a polymeric resin and a blowing agent,which provides the polymeric resin with a cellular structure uponexiting from the extruder. The extruder has an annular die with anannular die opening and a choke ring having an internal opening formedby an annular choke ring surface. The internal opening is sized toreceive at least a portion of the annular die. The annular die defines alongitudinal axis from which the annular die opening is spaced a firstradial distance and the annular choke ring surface is spaced a secondradial distance, with the difference between the second and first radialdistances defining a choke ring gap. The method includes forming theextrudate by mixing a polymeric resin and a blowing agent and thenextruding the extrudate through the annular die opening at a rate sothat the extrudate leaving the annular die opening contacts the annularchoke ring surface within a contact time of 1.0-20.0 milliseconds (ms).

The extrudate is preferably kept constrained against the choke ringsurface for a constrainment time of 5-7 ms and preferably between 8-50ms. The blowing agent can be a blend but is preferably 100% carbondioxide. The rate, i.e. the line speed, at which the extrudate is pulledthrough the annular choke ring opening can range from 50 mm/second to250 mm/second. It is preferred that the contact time be less than 8 msand more preferably be 5 ms or less.

In an alternate form of the method, the polymeric foam is formed bymixing a polymeric resin and a blowing agent consisting substantially of100% carbon dioxide, extruding the extrudate from the annular dieopening and through the choke ring gap, and controlling the extrudedfoam so it has an average cell size of 0.20 mm to 0.25 mm.

In another alternate form, the method relates to forming the extrudateby mixing a polymeric resin and a blowing agent comprising a blowingagent blend and extruding the extrudate through the choke ring gap.

The invention also relates to a foam, preferably made in accordance withthe above methods, having an average cell size of 0.20 mm-0.35 mm. Theaverage cell size is preferably less than 0.30 mm. Such a foam will haveno visible corrugation. Preferably, the foam will be corrugation-free.The foam can be made in a foam sheet, which has a thickness preferablyof 1-4 mm.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating the major functional componentsof a prior art foam extrusion system, including a choke ring surroundingan extrusion die;

FIG. 2 is an enlarged sectional view of the prior art choke ring and dieof FIG. 1, illustrating the relationship between the choke ring, die,and sizing mandrel of the apparatus and the corresponding interaction ofthe foam;

FIG. 3 is a view similar to FIG. 2 and illustrating a choke ring and dieaccording to the invention; and

FIG. 4 is an enlarged sectional view showing the relationship betweenthe choke ring and die according to the invention.

DETAILED DESCRIPTION

The invention is both a corrugation-free foam and an apparatus formaking the corrugation-free foam. The corrugation-free foam ispreferably made using the prior art tandem extruder with an improvedchoke ring alone or in combination with an improved die. The improvementin the choke ring permits the production of a polystyrene foam with 100%carbon dioxide blowing agent having a fine cell structure with theaverage cell size being between 0.20 mm and 0.35 mm. It heretofore wasthought impossible to produce such a corrugation-free foam with such afine cell size by using carbon dioxide as a blowing agent.

FIGS. 3 and 4 illustrate the improved choke ring 30 along with the newannular die 32. The choke ring 30 and annular die 32 are disclosed inthe context of the prior art tandem extruder configuration of FIG. 1.Therefore, similar parts in the figures will be identified by the samenumerals.

The choke ring 30 has an annular inner surface 31 which isconcentrically oriented with respect to a longitudinal axis 44 so thatthe inner surface 31 is located a radial distance r_(c) from thelongitudinal axis 44. Similarly, the annular die 32 is concentricallyoriented with respect to longitudinal axis 44 and the die opening 38 ispositioned a radial distance r_(d) from the longitudinal axis 44. Thedifference between the radial distances, r_(c)−r_(d), is preferably lessthan 4.57 mm.

Unlike the prior art dies, the die 32 according to the invention doesnot taper from the base to the tip of the die. Instead, the annular die32 has an outer periphery with an annular die opening 38 located in theouter periphery and forming an outlet for the extruder. The outerperiphery of the extrusion die 32 is located at the annular die opening38. The die 32 begins at its base with a generally constant crosssection portion 34, which transitions into an outwardly directedradially converging portion or collar 36, in whose apex the annular dieopening 38 is formed. The die then ends with a slightly tapered crosssection portion 40. The advantage of the die 32 over the prior art die12 is that the die outlet opening forms the largest outer diameter ofthe die 32; no other portion of the die 32 can interfere with theinsertion of the die into the choke ring opening defined by the chokering inner surface 31, especially for the very small gap sizes requiredby the invention.

The choke ring 30 preferably comprises a smooth, temperature-regulatedsurface concentric to the annular foam die positioned in such a manneras to direct but not reduce the flow of the foamable extrudate as itleaves the die. Preferably an adjustment mechanism of the choke ringapparatus allows the smooth, temperature-regulated surface 31 to bereproducibly positioned along the extrusion direction axis and be heldconcentric with the exit of die 32. An example of a suitable adjustmentmechanism is one that can adjust the up/down, side-to-side, andupstream/downstream position of the choke ring relative to the annulardie 32. Worm gears or similar mechanism can be used to make theadjustments.

It is also contemplated that the inner surface of the choke ring of thepresent invention can be constructed from any material that is solid atthe temperature of the foamable extrudate. It is further contemplatedthat the inner surface of the choke ring can be constructed of anysintered material. The preferred materials of construction for the chokering are thermoplastic polymeric materials with a glass transitiontemperature or melting temperature above about 160 C., thermosetpolymeric materials, and metallic materials. Examples of suitablethermoplastic materials include polytetrafluoroethylene, polyacetal,polyamides, polyesters, and polyoxymethylene, and crosslinkedpolyolefins. Examples of thermoset materials include phenolics and epoxyresins. Examples of metallic materials include aluminum, carbon steel,and stainless steel. It is contemplated that materials of higher thermalconductivity are more effective as materials of construction of theinner surface of the choke ring. The most preferred material ofconstruction for the inner surface of the choke ring is aluminum.

It is contemplated that the inner surface of the choke ring of thepresent invention can be configured with any suitably-shaped curve thatwill allow the expanding foamable composition to remain continuouslyconstrained by the temperature-regulated surface for a time period offrom about 5 to about 75 ms. The choke ring inner surface can preferablybe configured to have diametrically opposing boundaries of the innersurface of the choke ring which are parallel lines, non-parallel lines,convex curves or concave curves. For examples in which the diametricallyopposing sides of the choke ring are parallel lines, the boundaries ofthe inner surface describe a cylinder. For examples in which thediametrically opposing sides of the choke ring are non-parallel, theboundaries of the inner surface describe a frustoconical section of acone.

The apex of the cone formed by a projection of the inner surfaceboundaries may be upstream or downstream of the die face. The term“converging angle” is used herein to describe the angle formed if theapex of the cone formed by the projection of the inner surfaceboundaries is downstream of the extruder. The term “diverging angle” isused herein to describe the angle formed if the apex of the cone formedby the projection of the inner surface boundaries is downstream of theextruder. In one example in which the diametrically opposing sides ofthe choke ring form a convex curve, the boundaries of the inner surfacedescribe a section of a paraboloid. The vertex of said paraboloid formedby projection of the choke ring inner surface boundaries may be upstreamor downstream of the die face. In one example in which the diametricallyopposing sides of the choke ring form a concave curve, the boundaries ofthe inner surface describe an inner section of a torus.

Preferred configurations for the inner surface 31 of the choke ring arefrustoconical sections with a diverging angle, cylinders, andfrustoconical sections with a converging angle. The converging angle ordiverging angle is measured using the extrusion direction axis as thebase. The most preferred configurations for the choke ring inner surfaceare a frustoconical section of a cone having a converging angle lessthan about 20°, a cylinder, and a frustoconical section of a cone havinga diverging angle less than about 30°. The most highly preferredconfigurations for the choke ring surface are a frustoconical section ofa cone having a converging angle less than 10°, a cylinder, and afrustoconical section of a cone having a diverging angle less than about10°.

It is contemplated that a downstream end 46 of the inner surface of thechoke ring can be any configuration that will allow the foam to beginradial expansion without tearing of the foam surface. Preferredconfigurations for the downstream end of the inner surface of the chokering are a radius in the range of from 0.4 mm to about 7.0 mm. The mostpreferred configuration for the downstream end 46 of the inner surfaceof the choke ring is a radius in the range of from about 1.5 mm to about3.5 mm.

It is further contemplated that the convective fluid used fortemperature regulation of the choke ring can be any fluid that isconventionally used for cooling or heating applications as long as saidfluid is not subject to thermal decomposition at the extrudatetemperature and said fluid will not react with the material ofconstruction of the choke ring. Examples of convective fluids includewater, ethylene glycol-water mixture in any proportions, and commerciallow viscosity thermal oils commonly used for heat transfer application.The preferred convective fluid for the temperature regulation of thechoke ring is water.

The support for the choke ring inner surface can be any support thatwill enable positioning of the inner surface to be concentric with theannular die. The support can be attached to the extruder frame, the diebody or the floor. The support is preferably attached to the body of thedie.

Broadly, the process of the present invention combines an alkenylaromatic polymer, a nucleating agent, a physical blowing agentconsisting of at least 15 mole percent carbon dioxide and optionally oneor more auxiliary physical blowing agents, and optional colorants andadditives in the extruder B to form a foamable alkenyl aromatic polymercomposition or extrudate. The foamable extrudate is pressurized above aparticular threshold pressure specific to the composition and releasedto an area of lower pressure through an annular die G. Thetemperature-regulated annular choke ring H is positioned so that, withina contact time period of about 20 ms or less after the foamableextrudate 13 exits the die G, the outer surface of the extruded material13 is forced into contact with the smooth inner surface 31 of the chokering 30 in a manner that deflects the outer surface of extrudate 13 butdoes not restrict the flow through the die 32 or damage the surface ofthe foam. The contact with the choke ring surface 31 is maintained for aconstrainment time period in the range of from about 5 to about 75 ms.The foam extrudate 13 is then allowed to expand freely in the radialdirection and is drawn at constant line speed over a mandrel 15 whichhas a radius of about 1.5 to about 6.0 times the radius of the annularchoke ring 10 to form a substantially corrugation-free alkenyl aromaticpolymer foam.

The corrugations sought to be eliminated or reduced by the invention arewell known in the art and comprise multiple parallel regions formed onthe extruded sheet that are oriented in the extrusion direction andwhich are apparent to the unaided eye of an observer through eithervisible light reflection or visible light transmission. The least severecorrugations are those in which the multiple parallel regions arevisible only by transmitted light. Moderate corrugations are those whichare visible by reflected light and may have slight localized thicknessvariations that are perceptible to human touch. Severe corrugations arethose which also result in significant thickness difference betweenwidths that are less than about 4 percent of the overall sheet width.Extremely severe corrugation describes the condition when adjacentparallel segments actually join together across the width direction toproduce an overlap or even “S” folded cross-section in the thicknessdirection.

Mechanical properties of solid materials, such as flexural stiffness andtensile strength, are directly relatable to the mass distribution of thesubstrate. Thus, mechanical properties of foam are likewise directlydependent on the amount of solid mass or the localized density of thefoam. Corrugated foam generally has localized variations in density.Consequently, since lower density means lower strength, corrugations arethus deleterious to the overall mechanical properties of the foam.

Looking at the process in more detail and according to one embodiment ofthe present invention, the process for producing substantiallycorrugation-free alkenyl aromatic polymer foam begins by feeding pelletsof an alkenyl aromatic polymer into the extruder hopper A. The polymeralong with 0.02 to about 2.0 weight percent of pelletized talcnucleating agent and 0 to about 2-percent of optional additives,colorants, and fire retardants are fed by gravity into the hopper A.(All weight percentages relate to the weight of the extrudate, unlessotherwise noted.) The polymeric-talc-additives mixture is conveyedthrough the hopper A into the first extruder B and heated at the heatingzone C to a temperature sufficient to form a polymeric-talc-additivesblend.

A physical blowing agent consisting preferably solely of carbon dioxideand optionally one or more members selected from the group consisting offully hydrogenated hydrocarbon blowing agents, partially fluorinatedblowing agents, and combinations thereof, is added at the injector D ofthe extruder in an appropriate ratio to the target density. Preferably,the carbon dioxide comprises 1 to 3 weight percent and is injected in aliquid state. The polymeric-talc-additives blend and physical blowingagent are thoroughly mixed in the mixing zone E, transferred to thesecond extruder, and subsequently cooled in a cooling zone F to atemperature sufficient to form foam. The cooled foamable extrudateconsisting of the polymeric-talc-additives-physical blowing agent isextruded through an annular die into a lower pressure region on thedownstream side of the die G.

It is worth noting that carbon dioxide as used herein refers tocommercially available carbon dioxide. Commercially available carbondioxide is not pure and contains some contaminants. It is preferable touse 100% carbon dioxide.

As shown in FIGS. 3 and 4, the expansion of the extruded foamablecomposition is restricted by the choke ring 30, while making contactwith the smooth temperature-regulated inner surface 31, which isconcentric with the annular die 32. The inner surface 31 is sized sothat the foamable composition contacts the inner surface of the chokering within a contact time period of between about 1 and 20 ms. The foamis drawn over the inner surface 31 of the choke ring 30 and held thereagainst it for a constrainment time period of between about 5 and 75 ms.The resulting foam 13 is then allowed to expand freely in a radialdirection in the form of an air bubble 14 of regulated pressure, drawnover a cylindrical sizing mandrel 15 that has a diameter that is 1.5 to6.0 times that of the annular die, and collected on any conventionalsheet collection device. The resulting alkenyl aromatic polymer foam ofthe present invention is free of visible corrugations. The foampreferably has no corrugations visible to the eye by light transmission.

The polymeric foams produced by the present invention are generally of adensity of from about 30 kg/m³ to 120 kg/m³. The polymeric foamsproduced by the present invention generally have an average cell sizefrom about 0.20 mm to about 0.35 mm. The polymeric foams are producedwith uniform and consistent physical properties. The polymeric foams aresubstantially free of even the least severe corrugations. The polymericfoams are light in weight and can be converted into plates, cups, andbowls. Other contemplated applications for the polymeric foams producedby the present invention include uses in insulation, toys, and lowimpact protective packaging applications.

The polymeric foam produced by the present invention preferably has athin cross-section in the thickness direction of the foamed structurethat is less than about 5 mm. The preferred dimension in the thicknessdirection of the foamed product is from about 1.0 to 4.0 mm.

The “average cell size” as used herein is defined as the mean of thecross direction cell size and the extrusion direction cell size. Theextrusion direction cell size and cross direction cell size are measuredin conformance to ASTM Method D3676. The average cell size is in therange of about 0.15 to about 0.60 mm. The most preferred average cellsize is in the range of about 0.20 to about 0.35 mm in order to obtainthe most commercially desirable combination of finish, strength, anddensity. The contact time is a characteristic time period required by anelement of the foamable composition on the outer surface of theexpanding foamed structure to travel the distance from the annular dieoutlet opening to the inner surface 11 of the choke ring 10. The contacttime is calculated by dividing the distance between the choke ring innersurface and the die outlet (in units of length such as mm) by the linespeed (in units of length/time such as mm/sec) used in drawing the foamover the mandrel.That is: t _(c0)=(r _(c) −r _(d))/Lwhere t_(c0) is the contact time (in sec),

-   -   r_(c) is the mean radius of the choke ring inner surface (mm),    -   r_(d) is the radius of the annular die at the die gap (mm), and    -   L is the mean line speed of downstream equipment (mm/sec)

The preferred contact time is from about 1.0 to about 20 ms. The mostpreferred contact time is from about 1.0 to 8 ms. The most highlypreferred contact time is from 1.0 to 5.0 ms. A preferred range of linespeeds is 50 to 300 mm/s, with the most preferred line speed being150-250 mm/s.

The constrainment time is preferably a characteristic time period thatan element of the foamable composition on the outer surface of thefoamed structure actually travels along the inner surface of the chokering. The constrainment time is calculated by dividing the length of thechoke ring inner surface that is downstream of the annular die exit (inunits of length such as mm) by the line speed (in units of length/timesuch as mm/sec) used in drawing the foam over the mandrel.That is: t _(c1) =l _(c) /Lwhere t_(c1) is the constrainment time (in sec),

-   -   l_(c) is the contact length of the choke ring inner surface        (mm),    -   L is the mean line speed of downstream equipment (mm/sec)

The preferred constrainment time is from about 5 ms to about 75 ms. Themost preferred constrainment time is from about 8 to about 50 ms. Apreferred range of line speeds is 50 to 300 mm/s, with the mostpreferred line speed being 150-250 mm/s.

The alkenyl aromatic polymer preferably includes polymers of aromatichydrocarbon molecules that contain an aryl group joined to an olefinicgroup with only double bonds in the linear structure, such as styrene,α-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene,α-ethylstyrene, α-vinylxylene, α-chlorostyrene, α-bromostyrene, vinyltoluene and the like. Alkenyl aromatics polymers include homopolymers ofstyrene (commonly referred to as polystyrene) and copolymers of styreneand butadiene (commonly referred to as impact polystyrene).

The contact time, constrainment time, the choke ring gap size, and theline speed are interdependent or, in other words, functions of eachother. Although the contact time and constrainment time are useful forquantifying operational range of the process for obtaining acorrugation-free foam according to the invention, testing has also shownthat the currently preferred choke ring gap of 0.03 inches producescorrugation-free foam with a 0.20-0.35 mm average cell size for thecurrent range of line speeds.

The polystyrene resin or polystyrenic material preferably includeshomopolymers of styrene, and styrene copolymers comprised of at least 50mole percent of a styrene unit (preferably at least about 70 molepercent) and a minor (i.e. less than 50 mole percent) proportion of amonomer copolymerizable with styrene. The term “polystyrene resin” or“polystyrenic material” as used herein also includes blends of at least50 percent by weight of the styrene homopolymer (preferably at leastabout 60 weight percent) with another predominantly styrenic copolymer.The physical blends are combined in a dry form after the blends havebeen polymerized.

The polystyrene resin that can be used in the polymeric mixture can beany of those homopolymers obtained by polymerizing styrene to a weightaverage molecular weight (M_(W)) of from about 100,000 to about 450,000(commonly referred to as crystal polystyrene), can be any of thosecopolymers obtained by polymerizing styrene and from about 3 to 20 molepercent butadiene to a weight average molecular weight (M_(W)) of fromabout 100,000 to about 350,000, or can be any of those graft copolymersobtained by polymerizing a blend of polymerized styrene upon a nucleusof styrene-butadiene rubber (SBR) to a weight average molecular weightof from about 100,000 to about 350,000 (commonly referred to as impactpolystyrene).

The preferred crystal polystyrenes are uncrosslinked homopolymers ofstyrene and have a melt flow index of from about 0.5 to about 15.0dg/min. as measured by ASTM D1238 (nominal flow rate at 200 C. and 689.5kPa). The most preferred crystal polystyrene is uncrosslinkedpolystyrene having a melt flow index from about 1.0 to 3.0 dg/min.

Impact polystyrenes are generally classified as medium impactpolystyrene (MIPS), high impact polystyrene (HIPS) or super high impactpolystyrene (S-HIPS). The butadiene level of the impact polymer ispreferably in the range of from about 3 to about 10 weight percent ofthe copolymer (polybutadiene and polystyrene). The most preferredbutadiene level is in the range of from about 5 to 8 weight percent ofthe copolymer. The impact polystyrene generally has a melt flow index ofless than about 25 dg/min., and preferably less than about 8 dg/min. Themost preferred impact polystyrenes are uncrosslinked HIPSs having a meltflow index of from about 2.2 to 3.2 dg/min. as measured by ASTM D1238(nominal flow rate at 200 C. and 689.5 kPa), and a Notched Izod Impactof from about 9 to about 13 kg-cm/cm as measured by ASTM D256. TheNotched Izod Impact is the energy required to break notched specimensunder standard conditions and is work per unit of notch. Therefore, ahigher Notched Izod Impact indicates a tougher material.

The alkenyl aromatic polymer of the present invention can be obtained byblending two or more alkenyl aromatic polymers. For example, blends ofcrystal polystyrene and impact polystyrenes such as crystal polystyreneand HIPS, may be blended to comprise the alkenyl aromatic polymer of thepresent invention.

The nucleating agent preferably includes any conventional or usefulnucleating agent(s) used to adjust the size of the cells in the foamedstructure to the target size desired. The term “cell size control agent”has also been used interchangeably in the art. The amount of nucleatingagent to be added depends upon the desired cell size, the selectedblowing agent, and the density of the alkenyl aromatic polymer foamcomposition. The nucleating agent is generally added in amounts of fromabout 0.02 to 2.0 weight percent of the polymeric composition.Nucleating agents may be inorganic or organic compounds and aregenerally available in a small particulate form.

Examples of inorganic nucleating agents include clay, talc, silica, anddiatomaceous earth. The preferred organic nucleating agents includethose compounds which decompose or react at the heating temperaturewithin the extruder to evolve gas. Examples of these preferred organicnucleating agents include polycarboxylic acids and alkali metal salts ofa polycarboxylic acid in combination with a carbonate or bicarbonate.Some specific examples of an alkali metal salt include, but are notlimited, to the monosodium salt of 2,3-dihydroxy-butanedioic acid(commonly referred to as sodium hydrogen tartrate), the monopotassiumsalt of butanedioic acid (commonly referred to as potassium hydrogensuccinate), the trisodium and tripotassium salts of2-hydroxy-1,2,3-propanetriccarboxylic acid (commonly referred to assodium and potassium citrate respectively), and the disodium salt ofethanedioic acid (commonly referred to as sodium oxalate). An example ofa polycarboxylic acid is 2-hydroxy-1,2,3-propanetricarboxylic acid(commonly referred to as citric acid). Some examples of a carbonate or abicarbonate include, but are not limited to, sodium carbonate, sodiumbicarbonate, potassium carbonate, and calcium carbonate.

It is contemplated that mixtures of inorganic and organic nucleatingagents can also be used in the present invention. The most preferrednucleating agent is talc. Talc is preferably added in a powder form, butmay also be added in a carrier. If added in a carrier, the talcconcentration is preferably between 20 to 60 weight percent in analkenyl aromatic polymer which is preferably a styrene homopolymer.

The physical blowing agent for this invention includes at least 15 molepercent, preferably at least 50 mole percent, carbon dioxide andoptionally one or more auxiliary physical blowing agents. The mostpreferred amount is 100 percent carbon dioxide to provide the greatestpositive environmental and safety characteristics. The carbon dioxideblowing agent can be used at a rate of about 0.1 to 4.0 weight percent,but preferably about 1.0 to about 3.0 weight percent, of the totalextruder feed rate.

The auxiliary physical blowing agent comprises at least 1 mole percent,and preferably at least 5 mole percent, but less than 85 mole percent ofthe total blowing agent. More than one auxiliary physical blowing agentsmay also be included.

Examples of auxiliary physical blowing agents include but are notlimited to organic physical blowing agents.

The organic auxiliary physical blowing agents preferably includesorganic chemical compounds that have boiling points less than about 37C. These organic compounds include, but are not limited to, fullyhydrogenated hydrocarbons and partially fluorinated hydrocarbons whichmay be considered to be flammable. Flammable as defined herein generallyincludes those materials having flashpoints less than about 37.8 C.

Examples of fully hydrogenated hydrocarbon blowing agents include theinitial members of the alkane series of hydrocarbons that contain up tosix carbon atoms. Preferably, the hydrogenated hydrocarbon blowingagents are not regulated by governmental agencies as being specificallytoxic to human or plant life under normal exposure. The preferred C₁-C₆alkane compounds include methane, ethane, propane, n-butane, isobutane,n-pentane, isopentane, n-hexane, and blends thereof. The most preferredfully hydrogenated hydrocarbon auxiliary physical blowing agents are theC₄-C₅ and blends thereof.

The preferred partially fluorinated hydrocarbons auxiliary physicalblowing agents are hydrofluorocarbon gases that have molecules whichcontain up to three carbon atoms without any other halogen atoms otherthan fluorine. These partially fluorinated auxiliary physical blowingagents may be flammable. The most preferred partially fluorinatedhydrocarbon auxiliary physical blowing agents are 1,1-difluorethane(HFC-152a) and 1,1,1-trifluoroethane (HFC-143a). It is also contemplatedthat 1,1-chloroethane (HFC-142b) and 1-1-dichloro-2-fluoroethane(HFC-141b) may be added as auxiliary blowing agents for non-regulatedinsulation applications.

The optional additives used in the process preferably provide specific,non-mechanical physical properties to the foamed product and do notinterfere with or influence the extrusion of the foamed product.Additives may constitute from about 0.05 to about 5 weight percent ofthe total polymeric foam rate. Examples of additives include but are notlimited to antistats, fire retardants, perfumes, ultra-violet (UV) lightabsorber, UV stabilizers, and infra-red light tracers.

Similarly, colorants used in the process preferably include specificadditives included solely for the purpose of providing a desired colorto the foamed product that is different from the natural color providedby the alkenyl aromatic polymer when foamed. Examples of colorantsinclude various pigments and color concentrates as known in the art,such as carbon black and titanium dioxide white. The preferred form ofcolorant for addition to the extruder is in a pellet consisting of about1 to about 40 weight percent of the color material compounded in analkenyl aromatic polymer such as polystyrene that may be different fromthe alkenyl aromatic polymer used for the foam. The most preferred formof colorant for addition to the extruder is in a pellet consisting ofabout 5 to about 20 weight percent of the color material compounded inthe same alkenyl aromatic polymer as used for the foam. The mostpreferred concentration of additive is from 0.2 to 2.0 weight percent ofthe total extruder flow rate.

INVENTIVE EXAMPLE 1

Pellets of a previously extruded mixture of Dart Polymers, Inc. PS101high heat crystal polystyrene (specific gravity of about 1.05 g/cm³ anda melt index (MI) of about 1.8 dg/min.) and an undetermined level,between 1.0 and 5.0 weight percent, of Phillips K-Resin(styrene-butadiene copolymer) was mixed with 0.50 weight percent ofHuntsman 27678 1 talc concentrate pellets. The pellet mixture was heatedto form a blend in a 32:1 L:D Battenfeld Gloucester Engineering Co.,Inc. 2.5-inch (35.3 cm) single-screw extruder operating at a screw speedof about 95 rpm. Pressurized commercial-grade carbon dioxide (31.0 MPa)was injected at a rate of 2.31 kg/hr. The polymer melt and carbondioxide were mixed and further heated to a melt temperature of about 209C. and pressurized to 26.9 MPa at the extruder discharge.

The heated mixture was then transferred through a heated pipe to asecond, larger 3.5 inch (89 mm) single-screw cooling extruder operatingat 25 rpm. Subsequently, the extrudate was cooled to a melt temperatureof about 149 C. and pressurized to about 21.4 MPa for delivery at about75 kg/hr into a 5.40-cm diameter annular die.

The extrudate is pulled from the die by downstream equipment which isoperating at about 244 mm/sec (9.61 inches/sec) and is drawn intocontact with a choke ring having an inner surface diameter of 5.55 cm(2.19 inches) and a choke ring gap size of 0.762 mm (0.03 inch), for acontact time of 3.13 ms. The choke ring temperature was regulated by theflow of cooling water which was maintained at 27 C. The foam remained incontact with the inner surface of the choke ring for a distance of about6.4 mm, resulting in a constrainment time of about 26 ms. The foam wasthen allowed to expand freely and was subsequently drawn over a mandrelto form a foam having a density of 59.0 kg/m³, an average thickness of2.22 mm, and an average cell size of 0.25 mm. The foam is free ofcorrugations visible to the unaided eye by light transmission.

INVENTIVE EXAMPLE 2

This example is similar to Inventive Example 1 with reduction of thetalc concentrate level to 0.25 weight percent and an increase of thecooling water temperature on the choke ring to 63 C. and a reduction ofthe line speed to 199 mm/sec.

The contact time was 3.83 ms. The constrainment time was about 32 ms.The foam was then allowed to expand freely and was subsequently drawnover a mandrel to form a foam having a density of 60.0 kg/m³, an averagethickness of 2.57 mm, and an average cell size of 0.35 mm. The foam isfree of corrugations visible to the unaided eye by light transmission.

INVENTIVE EXAMPLE 3

This example is similar to Inventive Example 2 with a change to ablowing agent blend comprising 80 mole percent carbon dioxide and 20mole percent Phillips Chemical Company commercial grade isopentane at atotal injection rate of 2.54 kg/hr. Other adjustments were a coolingextruder screw speed of 26 rpm, choke ring cooling water temperature to32 C. and line speed to 203 mm/sec.

The contact time was 3.75 ms. The constrainment time was about 31 ms.The foam was then allowed to expand freely and was subsequently drawnover a mandrel to form a foam having a density of 60.6 kg/m³, an averagethickness of 2.56 mm, and an average cell size of 0.31 mm. The foam isfree of corrugations visible to the unaided eye by light transmission.

INVENTIVE EXAMPLE 4

This example is similar to Inventive Example 3 with a change to ablowing agent blend comprising 62 mole percent carbon dioxide and 38mole percent Phillips Chemical Company commercial grade isopentane at atotal physical blowing agent injection rate of 2.27 kg/hr. The linespeed was changed to 221 (8.70 inches/sec).

The contact time was 3.45 ms. The constrainment time was about 29 ms.The foam was then allowed to expand freely and was subsequently drawnover a mandrel to form a foam having a density of 82.2 kg/m³, an averagethickness of 1.78 mm, and an average cell size of 0.31 mm. The foam isfree of corrugations visible to the unaided eye by light transmission.

INVENTIVE EXAMPLE 5

This example is similar to Inventive Example 1 with reduction of thecarbon dioxide rate to 1.95 kg/hr, an increase of the choke ring coolingwater temperature to 32 C. and a reduction of the line speed to 156mm/sec (6.14 inches/sec).

The contact time was 4.89 ms. The constrainment time was about 41 ms.The foam was then allowed to expand freely and was subsequently drawnover a mandrel to form a foam having a density of 70.1 kg/m³, an averagethickness of 2.87 mm, and an average cell size of 0.30 The foam is freeof corrugations visible to the unaided eye by light transmission.

COMPARATIVE EXAMPLE 6

This example is similar to Inventive Example 2 with the elimination ofthe choke ring and change of the nucleating agent from a talcconcentrate to a powdered talc. The extrusion rate was 80.7 kg/hr.

The foam was allowed to expand from the die freely and was subsequentlydrawn over a mandrel to form a foam having a density of 59.9 kg/m³, anaverage thickness of 3.25 mm, and an average cell size of 0.59 mm. Thefoam has large cells and moderate corrugations that have visiblethickness variations on the surface of the foam.

COMPARATIVE EXAMPLE 7

This example is similar to Comparative Example 6 with a change of thenucleating agent to Boehringer Ingelheim Hydrocerol Compound (a mixtureof citric acid and sodium bicarbonate in a proprietary carrier).

The foam was allowed to expand from the die freely and was subsequentlydrawn over a mandrel to form a foam having a density of 85.7 kg/m³, anaverage thickness of 1.49 mm, and an average cell size of 0.34 mm. Thefoam has large cells and severe corrugations that have visible thicknessvariations on the surface of the foam.

COMPARATIVE EXAMPLE 8

This example is similar to Inventive Example 3 with the elimination ofthe choke ring. The foam was allowed to expand from the die freely andwas subsequently drawn over a mandrel to form a foam having a density of74.7 kg/m³, an average thickness of 1.84 mm, and an average cell size of0.35 mm. The foam has large cells and moderate corrugations that havevisible thickness variations on the surface of the foam.

COMPARATIVE EXAMPLE 9

This example is also similar to Comparative Example 8 with a change ofcarbon dioxide/isopentane mole fraction ratio from 80/20 to 70/30. Thefoam was allowed to expand from the die freely and was subsequentlydrawn over a mandrel to form a foam having a density of 79.8 kg/m³, anaverage thickness of 1.63 mm, and an average cell size of 0.29 mm. Thefoam has large cells and moderate corrugations that have visiblethickness variations on the surface of the foam.

COMPARATIVE EXAMPLE 10

This example is similar to Inventive Example b 1 except that the chokering had an inner surface diameter of 66.04 mm, resulting in a gap of 6mm (0.24 inches) and a contact time of 38 ms. The resultant foam wascorrugation-free and had a thickness of 2.91 mm, a density of 66.4kg/m², and an average cell size of 0.42 mm.

COMPARATIVE EXAMPLE 11

This example is similar to Inventive Example 1 except that a choke ringhaving an inner surface diameter of 63.12 mm, resulting in a gap of 4.6mm (0.18 inches) and a contact time of 20 ms, was used. The resultantfoam has severe corrugation and had a thickness of 2.46 mm, a density of54.1 kg/m³, and an average cell size of 0.26 mm.

COMPARATIVE EXAMPLE 12

Comparative Example 12 is similar to Comparative Example 11 except thatthe contact time was 30 ms, which resulted in a foam having severecorrugation with a thickness of 3.08 mm, a density of 65.8 kg/m², and anaverage cell size of 0.27 mm.

The key results of the examples are summarized in Table 1.

TABLE 1 Physical Blowing Agent CO₂ Isopentane Extruded Average ExampleMole Mole Density Cell Size Thickness Number Fraction Fraction (kg/m³)(mm) (mm) Corrugation INVENTIVE FOAMS 1 100% 0% 59.0 0.25 2.22 None 2100% 0% 60.0 0.35 2.57 None 3  80% 20%  60.6 0.31 2.56 None 4  62% 38% 82.2 0.27 1.78 None 5 100% 0% 70.1 0.30 2.87 None COMPARATIVE FOAMS 6100% 0% 59.9 0.59 3.25 Moderate 7 100% 0% 85.7 0.34 1.49 Severe 8  80%20%  74.7 0.35 1.84 Moderate 9  70% 30%  79.8 0.29 1.63 Moderate 10 100%0% 66.4 0.42 2.91 None 11 100% 0% 54.1 0.26 2.46 Severe 12 100% 0% 65.80.27 3.08 Severe

As can be seen, the invention with its choke ring and reduced contacttimes result in a highly desirable commercial foam having a cell sizewithin the preferred range of 0.20 to 0.35 without any corrugation,regardless of the percentage of carbon dioxide. The invention is furtheradvantageous in that the blowing agent can comprise 100 percent CO₂which has superior environmental and safety characteristics as comparedto the hydrocarbon and hydroflurocarbon blowing agents. The radialdistance r_(c) for a choke ring providing an acceptable foam is lessthan 30 mm, preferably less than 28 mm. The line speed is less than 300mm/second, preferably between about 150-250mm/s. The prior art foamsmade with a choke ring having contact times greater than 20 ms did notproduce a suitable foam because either the corrugation or cell size wastoo great. The prior art foams made without a choke ring were notsuitable because both the corrugation and the cell size were too great.

While the present invention has been described with reference to one ormore particular embodiments, those skilled in the art will recognizethat many changes may be made thereto without departing from the scopeand intentions of the present invention. Those variations thereof arecontemplated to fall within the scope and intention of the describedinvention.

1. A method of extruding a polymeric foam from an extrudate comprising apolymeric resin and a blowing agent for providing the polymeric resinwith a cellular structure upon exiting from an extruder having anannular die with an annular die opening and a choke ring having a chokering aperture formed by an annular choke ring surface and sized toreceive at least a portion of the annular die, the annular die defininga longitudinal axis from which the annular die opening is spaced a firstradial distance and the annular choke ring surface is spaced a secondradial distance, the method comprising: forming the extrudate by mixinga polymeric resin and a blowing agent; and extruding the extrudatethrough the annular die opening so that the extrudate leaving theannular die opening contacts the annular choke ring surface within acontact time of 1.0 to 20.0 milliseconds (ms).
 2. The method of claim 1and further comprising the step of constraining the extruded foam incontact with the choke ring for a constrainment time of 5 to 75 ms. 3.The method of claim 2 wherein the constrainment time is 8 to 50milliseconds.
 4. The method of claim 1 wherein the blowing agent issubstantially 100% carbon dioxide.
 5. The method of claim 2 wherein theblowing agent is a blend comprising carbon dioxide.
 6. The method ofclaim 5 wherein the blend comprises carbon dioxide and pentane.
 7. Themethod of claim 6 wherein the blend comprises less than 50% pentane. 8.The method of claim 1 wherein the extrudate is pulled away from the dieat a line speed of 50 mm/s to 250 mm/s.
 9. The method of claim 1 whereinthe polymeric resin is a styrenic resin.
 10. The method of claim 9wherein the styrenic resin is polystyrene.
 11. The method of claim 1wherein the contact time is less than 8.0 ms.
 12. The method of claim 11wherein the blowing agent is substantially 100% carbon dioxide.
 13. Themethod of claim 12 wherein the step of forming the extrudate includesmixing a nucleating agent into the extrudate prior to the extrudingstep.
 14. The method of claim 1 wherein the resultant foam has novisible corrugation.
 15. A method of extruding a polymeric foam from anextrudate comprising a polymeric resin and a blowing agent for providingthe polymeric resin with a cellular structure upon exiting from anextruder having an annular die with an annular die opening and a chokering having a choke ring opening formed by an annular choke ring surfaceand sized to receive at least a portion of the annular die, theextrusion die defining a longitudinal axis from which the annular dieopening is spaced a first radial distance and the annular choke ringsurface is spaced a second radial distance wherein the gap between thechoke ring surface and the die is equal to the difference between thesecond radial distance and the first radial distance, the methodcomprising: forming the extrudate by mixing a polymeric resin and ablowing agent consisting substantially of 100% carbon dioxide; andextruding the extrudate through the choke ring gap so that the resultantextruded foam has an average cell size of 0.20 to 0.35 millimeter (mm).16. The method of claim 15 wherein the extruding step yields a foamhaving an average cell size of less than 0.30 mm.
 17. The method ofclaim 16 wherein the polymeric resin is polystyrene.
 18. The method ofclaim 15 wherein the extruding step includes pulling the extrudatethrough the die opening at a rate so that the extrudate contacts thechoke ring surface within a contact time of 1.0 to 20.0 milliseconds(ms) after leaving the annular die opening.
 19. The method of claim 18wherein the contact time is 1.0 to 8.0 ms.
 20. The method of claim 15wherein the extruding step yields a foam having an average cell size ofless than 0.30 mm.
 21. The method of claim 20 wherein the foam has novisible corrugation.
 22. The method of claim 21 wherein the foam issubstantially corrugation-free.
 23. The method of claim 15 wherein thefoam is in sheet form.
 24. The method of claim 23 wherein the foam sheethas a thickness between 1.0 to 4.0 mm.
 25. The method of claim 24wherein the foam sheet has a density of 30 to 120 kg/m³.
 26. The methodof claim 15 and further comprising the step of extruding the extrudatethrough the annular die opening so that the extrudate leaving theannular die opening contacts the annular choke ring surface within acontact time of 1.0 to 20.0 milliseconds.
 27. The method of claim 26wherein the foam has no visible corrugation.
 28. The method of claim 26wherein the contact time is 1.0 to 8.0 milliseconds.
 29. The method ofclaim 15 wherein the foam has no visible corrugation.
 30. The method ofclaim 15 and further comprising the step of constraining the extrudedfoam in contact with the choke ring for a constrainment time of 5 to 75milliseconds.
 31. The method of claim 15 and further comprising the stepof blending pentane with the carbon dioxide to form the blowing agent.32. The method of claim 15 and further comprising the step of pullingthe extrudate away from the die at a line speed of 50 millimeters persecond to 250 millimeters per second.
 33. The method of claim 15 andfurther comprising the step of mixing a nucleating agent into theextrudate prior to the extruding step.
 34. A method of extruding apolymeric foam from an extrudate comprising a polymeric resin and ablowing agent for providing the polymeric resin with a cellularstructure upon exiting from an extruder having an annular die with anannular die opening and a choke ring having a die aperture openingformed by an annular choke ring surface and sized smaller to receive atleast a portion of the annular die, the extrusion die defining alongitudinal axis from which the annular die opening is spaced a firstradial distance and the annular choke ring surface is spaced a secondradial distance wherein the gap between the choke ring and the die isequal to the difference between the second radial distance and the firstradial distance, the method comprising: forming the extrudate by mixinga polymeric resin and a blowing agent comprising a blowing agent blend;and extruding the extrudate from the die opening and through the chokering gap of less than 4.57 mm.
 35. The method according to claim 34wherein the step of selecting the choke ring gap comprises selecting thechoke ring gap less than 0.8 mm.
 36. The method according to claim 34wherein the extruding step further comprises pulling the extrudate fromthe die opening at a line speed so that the extrudate leaving theannular die opening contacts the annular choke ring surface within acontact time of 1.0 to 20.0 milliseconds.
 37. The method of claim 36 andfurther comprising the step of constraining the extruded foam in contactwith the choke ring for a constrainment time of 5 to 75 milliseconds.38. The method of claim 37 wherein the constrainment time is 8 to 50milliseconds.
 39. The method of claim 34 wherein the extruding stepincludes extruding the extrudate through the choke ring gap such thatthe resultant foam has no visible corrugations.